Magnetically detectable plastic pipe

ABSTRACT

A magnetically detectable plastic, e.g., polyethylene, polyvinyl, etc., pipe for underground use comprises a hollow tubular plastic pipe having particles of magnetic material embedded in plastic and secured integrally with the wall of the pipe. The magnetic particles are of iron oxide or barium ferrite and of a size, shape, distribution and proportion such that the plastic pipe may be easily detected by magnetic detection apparatus on the surface when the plastic pipe is buried at a selected depth under the ground. The magnetic particles are uniformly distributed throughout the thickness and uniformly distributed around the entire circumference of the plastic pipe or the magnetic particles may be distributed in discrete portions of the plastic pipe, as for example, a uniform outer layer of the pipe or in stripes or strips extending along the pipe. The particles may be extruded with the plastic in forming the pipe, whether uniformly dispersed or in discrete regions or the pipe may be formed of plastic free of the magnetic particles and the magnetic particles provided in a separately applied strip which may be coextruded or fused or adhered to the wall of the pipe. The magnetic strips or magnetic portions of the plastic pipe may have selective areas or portions magnetized to encode information readable from the surface of the ground above the buried pipe or in inventorying pipe on the surface.

This is divisional of copending U.S. application Ser. No. 07/452,195filed Dec. 18, 1989.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates generally to facilitating the detection of pipesintended for deployment below ground, so that they may readily belocated, and relates more particularly to the development of a means andmaterial for the magnetic detection of plastic pipe.

BRIEF DESCRIPTION OF THE PRIOR ART

In many situations it is desirable to locate pipes below the surface ofthe earth, either to service or repair them or to avoid inadvertentdamage when subsequently excavating in the vicinity. Much of the pipeused in construction today, particularly for natural gas lines, isplastic rather than metal.

Plastic pipe is typically polyethylene, but may also be otherformulations such as polyvinylchloride (PVC). Plastic pipe has severaladvantages. It is extremely durable and reliable, relativelylightweight, easy to make, and cheaper than most other materials thatmay be used. Perhaps most importantly, it essentially inert, such thatnothing sent through it will react with it, unlike some other materials.

One problem in using plastic pipe, however, is that once buried it isdifficult to find, thus making it difficult to service the pipe and morelikely that unintentional damage will result from nearby digging.

Various attempts have been made to solve this problem. Ribbons have beenburied above the pipes, so that one could in theory locate the pipe bydigging until the ribbon, of a color contrasting to the soil, was found.See, e.g., Allen, U.S. Pat. No. 3,115,861, and Prosser, U.S. Pat. No.3,282,057. However, since two separate items must be buried, it isdifficult to locate the ribbon accurately over the pipe, and the ribbonmay "drift" away from the pipe after it is buried, thus making it moredifficult to locate the pipe even if the ribbon is found. Also, theribbon must later be located by digging as there is no way to detect itfrom above ground. Finally the ribbons may break or get lost in the soilbeing excavated, particularly where substantial digging is done, such asby heavy equipment, and may thus be lost before they can be located.

Similarly, metal foils have also been buried above pipes, as in Allen,U.S. Pat. No. 3,504,403, and Allen et al., U.S. Pat. No. 3,633,533, thealleged advantage being that these may be detectable from the surface bya metal detector device. Again, however, the problems of accurateplacement of the ribbon over the pipe and the possible drift render thissolution unsatisfactory.

A variation on this theme is shown in Southworth et al., U.S. Pat. No.3,568,626, which describes reinforcing a metal foil with some othermaterial, such as nylon, fiberglass or steel, so that the foil is noteasily broken, and using shorter portions or stretchable portions sothat an equipment operator will hopefully see a portion uncovered by theexcavation before the pipe is damaged. The metal foil may contain eithermagnetic or radioactive material so that once that part of the foil islocated by digging, the remainder can be followed from above ground byan appropriate detector. Again, however, the foil must be buried overthe pipe, and problems of drift and breakage remain. As with coloredribbons, it is possible that the operator will not see the foil beforestriking the pipe.

Keene, U.S. Pat. No. 4,573,829 involves a somewhat similar concept, inwhich a wire is placed inside the pipe. The wire can be detected eitherby a magnetometer, or if the wire is magnetic, or by applying an R.F.signal to the wire and locating it with an R.F. sensing device. However,just as the ribbons described above require an extra step to bury, anextra step is required here to insert the wire into the pipe duringconstruction of the pipeline. Also, if it is desirable to apply the R.F.signal to the wire, the ends must be located to do so. Finally, it ispossible that the wire may break, thus preventing detection by R.F.methods, since an open circuit would result and no signal could beapplied in such a case.

There have been other efforts to mark the location of an undergroundpipe, such as placing markers in the ground or inscribed in the pavementnear the location of the pipe. These are generally not accurate enoughto solve the problem, and much time can be wasted in locating the pipeeven when there is a marker in the general vicinity.

It is felt that none of these methods solve the problem of allowing fora plastic pipe to be buried in a single step, without additionalinconvenience or having to bury an additional object, while permittingdetection from above ground after burial by ordinary means and withoutlocating a buried ribbon or foil or the ends of a wire.

SUMMARY OF THE INVENTION

One of the objects of this invention is to provide a magneticallydetectable plastic pipe whose position can be easily detected from thesurface when buried in the ground.

Another object of this invention is to provide a magnetically detectableplastic pipe wherein magnetic particles are distributed in the wall ofthe pipe so that it is easily detected from the surface when buried inthe ground.

Another object of this invention is to provide a magnetically detectableplastic pipe wherein magnetic particles are distributed in the wall ofthe pipe by extrusion with the plastic at the time the pipe is formed sothat it is easily detected from the surface when buried in the ground.

Still another object of this invention is to provide a magneticallydetectable plastic pipe wherein magnetic particles are distributed indiscrete regions of the wall of the pipe so that it is easily detectedfrom the surface when buried in the ground.

Still another object of this invention is to provide a magneticallydetectable plastic pipe wherein magnetic particles are distributed instrips or stripes extending along the wall of the pipe so that it iseasily detected from the surface when buried in the ground.

Yet another object of this invention is to provide a magneticallydetectable plastic pipe wherein magnetic particles are distributed instrips or stripes extending along the wall of the pipe so that it iseasily detected from the surface when buried in the ground, the stripesor stripes being coextruded with the pipe at the time the pipe is formedor extruded separately from the pipe and joined to the pipe by adhesiveor by being fused to the wall of the pipe.

Yet another object of this invention is to provide a magneticallydetectable plastic pipe wherein magnetic particles are distributed in oron the wall of the pipe and selected areas or portions magnetized toencode data that it is easily detected and read from a distance.

Other objects of the invention will become apparent from time to timethroughout the specification and claims as hereinafter related.

The present invention provides several methods of rendering plastic pipemagnetically detectable, all of which involve mixing magnetic materialwith polyethylene or other suitable carriers. One embodiment comprisesthe steps of selecting magnetic materials suitable due to their shape aswell as strength of magnetic remanence and cost, formulating a mixtureof unformed plastic and magnetic material and mixing the materials,extruding said mixture into various sizes of plastic pipe.Alternatively, the concentration of magnetic material may besignificantly increased, and strips of highly magnetic plastic or othermaterials may either 1) be coextruded with the plastic pipe, or 2)formed or extruded alone and later joined to the pipe by heating bothpipe and strip or by application of adhesives.

This invention has several advantages. Since the pipe itself, in one ofthe above ways, contains the magnetic signal, the person or personsinstalling the pipe need only bury the pipe, and not any other ribbons,wires or foils. Nor do any wires need to be inserted into the pipeduring the installation. No special care is required, either, in thejoining of sections of pipe. Once buried, there are no problems with thematerials containing the locating (magnetic) signal drifting withrespect to the location of the pipe. The pipe is thus detectable fromabove ground by normal magnetometers, and thus there is no need tolocate a ribbon or foil during excavation, or to locate a wire to applyan R.F. signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the extrusion or coextrusion of plastic pipecontaining particles of magnetic material therein.

FIG. 2 is a longitudinal section of an extruded plastic pipe containingparticles of magnetic material dispersed uniformly in the wall thereof.

FIG. 3 is a lateral section of an extruded plastic pipe containingparticles of magnetic material dispersed uniformly in the wall thereof.

FIG. 4 is a lateral section of an extruded plastic pipe containingparticles of magnetic material dispersed in the outer section of thewall thereof.

FIG. 5 is a lateral section of an extruded plastic pipe containingparticles of magnetic material dispersed uniformly in strips of the wallthereof.

FIG. 6 is an exploded, isometric view of a plastic pipe with magneticmaterial in plastic strips positioned for assembly on the pipe inaccordance with one embodiment of this invention.

FIG. 7 is an isometric view of a plastic pipe with magnetic in plasticstrips secured on the pipe in accordance with one embodiment of thisinvention.

FIG. 8 is a cross section taken on the line 8--8 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention comprises a magnetically detectable plastic, e.g.,polyethylene, polyvinyl, etc., pipe for underground use which comprisesa hollow tubular plastic pipe having particles of magnetic, preferablyneedle-like particles or platelets of iron oxide or barium ferrite,material embedded in plastic and secured integrally with the wall of thepipe. The magnetic particles are of iron oxide or barium ferrite and ofa size, shape, distribution and proportion such that the plastic pipemay be easily detected by a magnetic detection apparatus on the surfacewhen the plastic pipe is buried at a selected depth, e.g., 3-5 feet,more or less, under the ground.

The magnetic particles may be uniformly distributed throughout thethickness and uniformly distributed around the entire circumference ofthe plastic pipe or may be distributed in discrete portions of theplastic pipe, as for example, a uniform outer layer of the pipe or instripes or strips extending along the pipe. The particles may beextruded with the plastic in forming the pipe, whether uniformlydispersed or in discrete regions or the pipe may be formed of plasticfree of the magnetic particles and the magnetic particles provided in aseparately applied strip which may be coextruded or fused or adhered tothe wall of the pipe. The magnetic strips or magnetic portions of theplastic pipe may have selective areas or portions magnetized to encodeinformation readable from the surface of the ground above the buriedpipe or otherwise at a distance from the pipe.

EXTRUSION OF PIPE WITH EMBEDDED MAGNETIC PARTICLES

Referring to the drawings, FIGS. 1-5 show a schematic or block diagramfor extruding pipe and various sectional views of extruded plastic pipewith embedded magnetic particles. In FIG. 1, a mixer receives a supplyof extrudable plastic 2, usually powder or pellets, and a supply ofmagnetic particles 3. The mixer is a conventional plastics extrusionmixer of the type for blending dyes or pigments or fillers prior toextrusion. The plastic used is polyethylene, although polyvinyl or othermoldable or extrudable organic resin polymers are satisfactory if themolded or extruded pipe has satisfactory strength. The magneticparticles are preferably needle-like particles or platelets of ironoxide or barium ferrite which will magnetize readily and hold magnetismsubstantially permanently.

The mixture of molten plastic and magnetic particles is fed from mixer 1to a plastics extruder 4 where it is extruded into a hollow tubular pipewith the magnetic particles distributed according the type of extrusiondie used. Extrusion dies are commercially available or described inpatents or technical literature in a variety of forms.

One such die is a single extrusion die for extruding a plastic withoutany filler or for extruding a plastic admixed uniformly with a filler,in this case magnetic particles. Another extrusion die has concentricextrusion for extruding one material over another to produce a compositeproduct having outer and inner portions of different composition or forlaying down a thin coating over a main extrusion product.

Still another extrusion die is one which has segmented outlets forextruding separate streams of plastic of different compositions. Thistype of die is used in extruding tubular films or flexible or rigidtubing with stripes by alternating streams of unpigmented plastic withstreams of pigmented plastic. Information of these dies is readilyavailable in the patent literature and in various texts and technicalarticles on plastics engineering, as well as in various manufacturer'sliterature.

The pipe 5, shown schematically in FIG. 1, may have magnetic particles 6uniformly distributed through the wall of the pipe as seen in FIGS. 2and 3. In FIG. 4, the pipe 5 is extruded from a concentric die forproducing a two (or more) layered pipe. In this embodiment, the pipe 5may have an inner portion 7 which is unpigmented plastic and an outerlayer 8 containing the magnetic particles 6. The layers 7 and may be ofnearly equal thickness, e.g., 0.125" each, or the inner layer 7 may bethe main thickness of the pipe wall, e.g., 0.250", and the outer layer 8a thin layer or coating of a thickness of the order of about 0.025". Thepipe 5 shown in FIG. 5 is from a split die extruding alternate streamsof unpigmented plastic and of plastic containing magnetic particles.This pipe has regions 9 of unpigmented plastic and regions 10 of plasticcontaining magnetic particles 6. In this embodiment, themagnetic-particle-containing regions 10 are preferably stripes runninglongitudinally of the pipe. The embodiments of FIGS. 4 and 5 may becombined, if desired, so that the stripes 10 do not extend through theentire thickness of the pipe 5 but are recessed in the surface, e.g.,strip 10a in FIG. 5

The pipes 5, in any of the several embodiments, are magnetized byexposure to a high-power magnet to magnetize the magnetic particles 6.This magnetization may be done before, during or after the extrusion ofthe pipe 5. Magnetizing the particles 6 prior to extrusion is notpreferred because the magnetized particles tend to adhere to each otherand are difficult to disperse. Magnetization during extrusion, aftermagnetic particles 6 have been dispersed, or after extrusion, after theplastic has hardened, is preferred. The amount of magnetic material inthe pipe is sufficient to produce the desired amount of magnetism todetect the pipe at a depth of 3-5' or more. A concentration of 8% ofmagnetic particles, e.g., iron oxide or barium ferrite, disperseduniformly in a 3" plastic pipe having a 0.230" wall is sufficient fordetection of the pipe at a depth of 3'. The amount and/or concentrationof the magnetic particles in the plastic pipe is varied, in practice,according to the size and wall thickness of the plastic pipe. Also, inembodiments such as those of FIGS. 4 and 5, the concentration of themagnetic particles is higher so that there is enough magnetic materialpresent to produce the desired magnetic effect. In such a case, anexperimental determination is made initially of the effect of loadingthe plastic with magnetic particles on tensile strength and the productdesigned to balance needed tensile properties with the amount ofmagnetic material required in the pipe.

APPLICATION OF MAGNETIC-PARTICLE CONTAINING STRIPS TO PIPE

In the embodiment of FIGS. 6-8, particles of a magnetic material such asiron oxide or barium ferrite are suspended in a polyethylene, or otherorganic polymer, matrix at a higher concentration and then formed orextruded as a tape. This tape is then affixed to the pipe.Alternatively, the tape may be coextruded with the pipe.

Magnetic products of this composite type currently exist in a number ofapplications. For example, an iron oxideloaded PVC product, manufacturedby Leon Plastics, is used as a high density plastic feedstock ininjection molding applications where "heavy" plastic parts are required.(PVC and polyethylene plastics have a density of 1 g/cc.)

A second magnetic composite is a flexible magnetic rubber strip productmanufactured by 3M Corporation. This product is used to make magneticsigns and emblems for use on automobiles and refrigerator doors. Insteadof iron oxide, however, the magnetism is provided by barium ferrite,which is magnetically similar to iron oxides in remanent magnetizationbut has higher coercivities.

Other products are manufactured both by 3M and other companies, in whichmagnetic particles are mixed with either polypropylene or nylon andinjection molded to form, for example, stator parts for motors or wheelsto be used in counters.

To determine the concentrations of magnetic material needed to mix withpolyethylene, experiments were first performed with high concentrationsof magnetic material, concentrations suitable for the application ofmagnetic strips to the sides of the pipe. These runs were made with asmall commercial mixing machine (Braybender Corp.). Polyethylene "fluff"mixed with various amounts of different types of magnetic particles wereheated and mixed in the Braybender and then pressed with several tons offorce into small strip molds. Ratios (by weight) of up to 2.3/1 bariumferrite/polyethylene and up to 1.6/1 gamma iron oxide/polyethylene weretested.

The resultant magnetic plastic composites indicate that the physicalproperties of the polyethylene are not prohibitively affected by theaddition of the magnetic oxides. Tensile strength did not differsignificantly from pure polyethylene, although elongation strength wasconsiderably lessened.

The samples were magnetized and measured; both types of magnetic oxideexhibited magnetizations of about 40 emu/cc. The two magnetic oxidesdiffered significantly in terms of the resultant direction ofmagnetization; barium ferrite would only magnetize across the flatdimension, i.e., thickness, of the strips, while iron oxide magnetizedonly in the plane, i.e., across the width, of the strip. This differenceis attributed to the difference in shape of the magnetic particles. Thebarium ferrite is small platelets whose easy direction of magnetizationis across the thickness of the plate, while the gamma iron oxide is longneedles whose easy direction of magnetization is along the length of theneedle.

Calculations were performed to determine the magnetic moment that wouldarise from adhering these magnetic strips to 2 inch diameter plasticpipe. As an example, a 3 foot length of pipe with four strips (0.75 inchwide by 0.025 inch thick) would have a total magnetization of 1760 emu(FIGS. 6-8). At a distance of three feet, the pipe exhibits a magneticfield of 230 gammas (1 gamma=10-5 Gauss; earth's magnetic field about50,000 gammas). Since pipe sections are nearly always longer than 3feet, larger magnetic signals are to be expected, since the pipe beyondthe 3 foot length of the example will also contribute to the magneticsignal.

The extrusion process causes the magnetic particles to be aligned duringforming. Further alignment is achieved by applying magnetic fieldsduring these formation processes. This significantly increases theresultant magnetization, by as much as nearly a factor of 2. Theliterature indicates that a different carrier will allow much greaterconcentrations of magnetic material and hence, much higher remanences,up to 350 emu/cc, to be achieved. This means that it is possible toachieve detection field strengths of 1000 gammas or more (as compared tothe 230 gammas of the experiment)

In the development of the embodiments of FIGS. 1-5, the extrusion of amagnetically doped polyethylene was investigated. Initially weightratios of 1.6:1 of magnetic oxide to polyethylene were used, both forbarium ferrite and iron oxide. This proved to be too much, as viscositywas too large for the small extruder being utilized. The ratio wasreduced to 0.5:1 and several hundred feet of tape (6" wide and 0.025"thick) of each composition was produced. Measurement of the magneticmoment indicated 12 emu/cc for both magnetic oxides, a value roughlyconsistent with the concentration of magnetic material used.

Four of these strips 11 containing magnetic particles 12 each wereadhered to 3 foot sections of five different pipe 13 diameters rangingfrom 1.3 to 4.5 inches (again in the configuration shown in FIG. 6-8).These produced magnetic signatures at a 3 foot distance, of 20 gammasfor the smallest pipe to 95 gammas for the largest. All measured valuesagreed perfectly with values calculated for each pipe diameter. Thismeans that the model being used to calculate the magnetic signatures isrealistic. The process has been described with reference to plastic butother organic polymers, including rubber, could be used.

The smaller concentration of magnetic material in the extrudedpolyethylene pipe bears on the process of adding the magnetic materialdirectly to the polyethylene used to make the pipe. Since the pipe wallvolume is much larger than the volume of the tapes discussed above,greater amounts of magnetic material will be present, giving rise tolarger signals than the extruded tape gave. Lower concentrations altersthe physical properties of the polyethylene to a significantly lesserextent than found in the strip-molded material.

Several conclusions can be made at this point. Heavily loaded magnetictapes can be used to magnetically "see" plastic pipe at depths of threeto five feet and greater. 2) Addition of iron oxides to the polyethyleneformulation used to make standard plastic gas pipe does not drasticallyalter the physical properties of this polyethylene, hence makespractical a process of making the plastic pipe itself magnetic 3) Choiceof iron oxides with the shape of the particles in mind allows muchhigher magnetizations to be achieved. This is due to alignments,variously different according to the process, induced by the formingprocess.

Finally, the cost of producing a magnetic pipe product will have animportant effect on its ultimate acceptance. Ideally the cost should bea small part of the overall pipe manufacturing cost. The data developedin connection with this process indicate that the process and resultingproduct are commercially feasible.

ENCODING OF DATA IN PIPE

The magnetic strips or magnetic portions of the plastic pipe may haveselective areas or portions magnetized to encode information readablefrom the surface of the ground above the buried pipe or otherwise at adistance from the pipe, as in inventorying, etc. The pipe, whetherformed with uniformly spaced magnetic particles or with stripes ofmagnetic particles in the wall of the pipe, or in separately formed orseparately applied strips of plastic containing magnetic particles, maybe selectively magnetized, i.e., in different areas or regions, or maybe magnetized to selected levels of magnetism. In practical use, thepipe with magnetic stripes or strips on or in the pipe wall isselectively magnetized in the manner of magnetic bar codes to encodeinformation which can be read from the surface of the ground when thepipe is buried. Such information may include such data as themanufacturer, type of pipe, installer, time of installation, etc.

While this invention has been described fully and completely withemphasis on several preferred embodiments, it should be understood that,within the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

I claim:
 1. A method of producing magnetically detectable plastic pipefor underground use, comprising;providing an organic plastic forextrusion, providing particles of magnetic material comprisingelectrically-nonconductive, highly-magnetizable iron oxide or bariumferrite, mixing said particles of electrically-nonconductive,highly-magnetizable iron oxide or barium ferrite uniformly with saidorganic plastic, heating and extruding said mixture of organic plasticand electrically-nonconductive, highly-magnetizable iron oxide or bariumferrite and highly magnetizing said particles to produce a hollowtubular pipe having said highly magnetized particles distributed andproportioned such that the plastic pipe may be easily detected bymagnetic detection apparatus on the surface when the plastic pipe isburied at three to five feet or more under the ground.
 2. A methodaccording to claim 1 in whichsaid electrically-nonconductive,highly-magnetizable iron oxide or barium ferrite particles areselectively magnetized after formation of said pipe to encode datatherein readable from the surface after the pipe is buried three to fivefeet or more under the ground.
 3. A method according to claim 1 inwhichsaid mixture of organic plastic and magnetic particles is extrudedto produce a hollow tubular plastic pipe which, saidelectrically-nonconductive, highly-magnetizable iron oxide or bariumferrite particles are uniformly distributed throughout the thickness ofthe wall of said plastic pipe, and uniformly distributed around theentire circumference of said plastic pipe.
 4. A method according toclaim 1 includingproviding an additional quantity of said organicplastic free of said magnetic particles, and extruding said organicplastic free of said magnetic particles and said mixture of organicplastic and electrically-nonconductive, highly-magnetizable iron oxideor barium ferrite particles together to produce a hollow tubular plasticpipe having said electrically-nonconductive, highly-magnetizable ironoxide or barium ferrite particles distributed and proportioned inseparate discrete regions of the plastic pipe such that the plastic pipemay be easily detected by magnetic detection apparatus on the surfacewhen the plastic pipe is buried at three to five feet or more under theground or otherwise at a substantial distance from the plastic pipe. 5.A method according to claim 1 includingproviding an additional quantityof said organic plastic free of said magnetic particles, and extrudingsaid organic plastic free of said magnetic particles and said mixture oforganic plastic and electrically-nonconductive, highly-magnetizable ironoxide or barium ferrite particles together to produce a hollow tubularplastic pipe having said electrically-nonconductive, highly-magnetizableiron oxide or barium ferrite particles distributed and proportioned in aseparate discrete region of the plastic pipe comprising an outer layerintegral with the plastic pipe such that the plastic pipe may be easilydetected by magnetic detection apparatus on the surface when the plasticpipe is buried at three to five feet or more under the ground orotherwise at a substantial distance from the plastic pipe.
 6. A methodaccording to claim 1 includingproviding an additional quantity of saidorganic plastic free of said magnetic particles, and extruding saidorganic plastic free of said magnetic particles and said mixture oforganic plastic and electrically-nonconductive, highly-magnetizable ironoxide or barium ferrite particles together to produce a hollow tubularplastic pipe having said electrically-nonconductive, highly-magnetizableiron oxide or barium ferrite particles distributed and proportioned inseparate discrete regions of the plastic pipe comprising stripes ofmagnetic material extending longitudinally of the plastic pipe such thatthe plastic pipe may be easily detected by magnetic detection apparatuson the surface when the plastic pipe is buried at three to five feet ormore under the ground or otherwise at a substantial distance from theplastic pipe.
 7. A method according to claim 6 in whichsaidelectrically-nonconductive, highly-magnetizable iron oxide or bariumferrite particles are magnetized in the course of forming said pipe. 8.A method according to claim 6 in whichsaid electrically-nonconductive,highly-magnetizable iron oxide or barium ferrite particles aremagnetized after formation of said pipe.
 9. A method according to claim6 in whichsaid electrically-nonconductive, highly-magnetizable ironoxide or barium ferrite particles are selectively magnetized afterformation of said pipe to encode data therein readable from the surfaceafter the pipe is buried or otherwise at a distance from the plasticpipe.
 10. A method of producing magnetically detectable polymeric pipefor underground use, comprising:providing a hollow tubular pipe oforganic polymer, providing a strip of organic plastic containingparticles of electrically-nonconductive, highly-magnetized iron oxide orbarium ferrite, affixing at least one said strip of organic polymercontaining particles of electrically-nonconductive, highly-magnetizediron oxide or barium ferrite to said plastic pipe to produce a compositepipe having said electrically-nonconductive, highly-magnetized ironoxide or barium ferrite particles positioned and proportioned such thatthe plastic pipe may be easily detected by magnetic detection apparatuson the surface when the plastic pipe is buried at a selected depth underthe ground or otherwise at a distance from the plastic pipe.
 11. Amethod according to claim 10 in whichsaid strip ofmagnetized-particle-carrying polymeric material is extruded andsubsequently fused to the surface of the wall of said plastic pipe. 12.A method according to claim 10 in whichsaid strip ofmagnetized-particle-carrying polymeric material is adhered to thesurface of the wall of said plastic pipe.
 13. A method according toclaim 10 in which said strip of magnetized-particle-carrying polymericmaterial is coextruded with said plastic pipe.
 14. A method according toclaim 10 in which said particles are magnetized in the course of formingsaid strip.
 15. A method according to claim 10 in whichsaid particlesare selectively magnetized after formation of said pipe to encode datatherein readable from the surface after the pipe is buried or otherwiseat a distance from the plastic pipe.
 16. A method according to claim 10in whichsaid particles are magnetized after formation of said strip. 17.A method according to claim 16 in whichsaid magnetic particles areselectively magnetized after formation of said pipe to encode datatherein readable from the surface after the pipe is buried.